1. Field of the Invention
This invention relates to a signal accumulating time control method and apparatus for a signal accumulating type radiation sensing device which produces an electrical signal corresponding to the amount of incident radiation and accumulates the same.
2. Description of the Prior Art
For example, in the use of a solid state image sensing device such as CCD which has been remarkably developed in recent years, it is known that the signal accumulating time thereof must be appropriately controlled in accordance with the amount of light incident on the light-receiving surface thereof to obtain a good image output. Various methods and apparatus for controlling the accumulating time have been proposed. For example, a feedback control method wherein the output level of the image sensing device is compared with a predetermined threshold (upper limit and lower limit) and on the basis of the result of the comparison, control of the accumulating time is effected in the next operation cycle has already been known in the art. In this method, along a predetermined plurality of separate accumulating times, the accumulating time is prolonged or shortened each time as required, by the result of one output read-out (that is, this is a step-by-step follow-up type control). However, in such control method, for example, in a case where it is necessary to change the accumulating time greatly as when the intensity of incident radiation fluctuates greatly, a considerably long time is required until an appropriate accumulating time corresponding to the amount of light incident on the light-receiving surface of the image sensing device is reached, and a problem in the responsiveness arises in a case where any system operation is effected after an image signal is obtained. Particularly, in a camera or the like using such a solid state image sensing device (not only for image sensing but also including a focus detecting device, for example), the time required until the normal operation of the system is performed against the great fluctuation of the intensity of incident radiation is a very important factor which determines the performance of the system, and the shorter this is, the more preferable it is.